1. Field of the Invention
The present invention is directed to a method and apparatus for detecting overheated wheel and axle components on railroad cars. More particularly, the present invention is directed to an infrared scanning circuit that employs analog and digital microelectronic circuitry in processing the infrared emitted from such components to determine, in conjunction with ancillary circuitry, whether any individual component is overheated, and, if so, to produce a warning signal that may be transmitted to any of a number of warning read-out devices.
2. The Prior Art
Modern railroad car wheel bearings are permanently lubricated sealed units designed to last for the life of the car. Sometimes, however, these wheel bearings fail during use, causing excess friction between the axle and the bearing and producing excess heat, resulting in a condition referred to as a hot box. Normally, the bearings operate at about 20 degrees centigrade (C.) above the ambient temperature. When a bearing begins running at more than about 70 degrees C. above the ambient temperature, it has already failed. If the car continues moving at the same speed, internal fracture of a roller bearing can cause the bearing to seize, creating thermal run-away. In thermal run-away the bearing temperature rises dramatically from about 20 degrees C. above the ambient temperature to more than 300 degrees C. above the ambient temperature in about one-half mile of travel; under further travel the bearing melts and falls off the axle; the wheels fall off; and the truck falls to the ground, uncoupling the car from those in front of it, triggering the emergency brakes on the whole train and causing the portion of the train behind the disabled car to collapse into an accordion-patterned wreck as the cars leave the tracks.
Brakes that fail to release also produce a dangerous condition that can cause a similar disaster. The affected wheel rises to temperatures on the order of 600 degrees C. and creates a condition known as a hot wheel. If unchecked, the wheel ultimately disintegrates and a derailment results.
Because the hot box and the hot wheel can be so dangerous, the railroad service industry has devoted significant resources to building detectors that automatically check passing trains for hot boxes and/or hot wheels. Such detectors are conventionally spaced along railroad tracks at about twenty to fifty mile intervals along main-line track throughout the United States, and many are necessarily located in remote places. In addition, detectors are continually exposed to and must operate in extremes of heat and cold, wind and rain, and vigorous vibration. Naturally the railroad industry needs highly reliable, low maintenance hot box and hot wheel detectors, preferably at reasonable cost. Although previous efforts have produced several sound products, a number of important problems have not been solved in the prior art.
Detectors in present use typically include a sensing unit lens for focusing infrared from passing wheels onto an infrared sensor and electrical circuitry to develop a signal that is representative of the journal or wheel temperature. One sensing unit is placed along one rail of the tracks and a second sensing unit is placed along the other rail of a set of tracks, so that both sides of a train can be monitored. Electrical lines connect these trackside sensing units to processing circuitry which is conventionally located in a "bungalow" close to the tracks. The final output signal of the detector can be used to create a written record of the temperature of each of the journals or wheels that passes the sensing units. In hot box detectors this signal triggers a warning output if the signal indicates that the temperature of a wheel journal exceeds a predetermined value (generally about 70 degrees C. above the ambient temperature), i.e., if a hot box is detected. The warning output can be used to stimulate any convenient type of warning device. For example, the warning can be displayed on a light board in the cab of the locomotive or in a dispatcher s office, or it can cause a stop signal to be displayed on traffic signals along the tracks.
The prior art includes the commonly used bolometer type of hot box and hot wheel detector. It employs temperature sensitive resistors (thermistors) in a bridge arrangement. Such units also require a highly stable and accurate high voltage supply. Because the signal-to-noise ratio of the bolometer decreases to unacceptable levels even within the normal operating temperature ranges of the detectors, automatic heaters must be installed to keep the thermistors warm enough to work properly. Once heaters are installed, it may become necessary to upgrade the optical system of the bolometer. Thus, overcoming the fundamental problems inherent in a bolometer greatly complicates the device, making it more expensive to build and maintain, and less reliable. In addition, the frequency response of the bolometer is narrower than desired, restricting the top speed a train may be traveling while the bolometer checks for hot boxes or wheels. For a more detailed examination of the shortcomings of bolometers, see U.S. Pat. No. 4,068,811, entitled "Hotbox Detector," issued Jan. 17, 1978.
In an effort to overcome these and other problems, pyroelectric cells were introduced for use as the infrared detection element in hot box and hot wheel detectors. Pyroelectric crystals acquire opposite electrical charges on opposite faces when subjected to a change in temperature. Pyroelectrical cells are also exhibit some piezoelectrical properties, but the incidence of spurious signals generated by vibration have been virtually eliminated through physically isolating the cell from vibration. Pyroelectrical cells overcome many of the difficulties associated with bolometers. For example, hot box detectors built around pyroelectric detection schemes cost only about one-fifth to one-half as much as bolometers. Because the pyroelectric cell generates its own electrical charge, large power supplies are not needed and the high impedance obviates the careful impedance matching of the bolometer. Further, no heaters are required because the signal-to-noise ratio is substantially flat over the required temperature range. Accordingly, simpler and cheaper optical systems can be used. Nevertheless, use of pyroelectric cells confronts the designer with other serious difficulties.
For example, pyroelectric cells tend to have an extremely poor voltage gain response to the impinging infrared, or heat signal, when considered over any reasonable range of signal input frequencies, that is, over a range of train speeds. The voltage gain response tends to depend on the length of time that the pyroelectric cell is exposed to the infrared, as well as the strength of the infrared. Thus, a typical infrared sensor employing a pyroelectric cell has an acceptably flat or constant voltage gain response over only about two percent of the frequency range required for acceptable hot detector operation, which is about 0.5 Hz to about 300 Hz. This prevents accurate temperature readings when a linear amplifier is used, yet only the voltage gain has a sufficiently high signal-to-noise ratio to provide a usable signal.
One prior art approach to overcoming this difficulty is to add a compensating signal to the pyroelectric cell signal to produce a signal having a flat frequency response over the normal range of frequencies, as set forth in the aforementioned U.S. Pat. No. 4,068,811. Over time, however, the breakpoint at which the voltage response of the pyroelectric cell begins to decline sharply drifts unpredictably due to changes in capacitance and response time. It may drift up or down the frequency scale; it may drift by different amounts. Neither the magnitude nor the direction of the drift will be the same for different detectors. The circuitry that develops the compensating signal cannot compensate for this drift, and so the detector will not produce the flat voltage response over the relevant frequency range that the remaining circuitry must have for proper operation. This long term signal drift requires frequent calibration checks of the pyroelectric cell. Such checks, and if necessary, re-calibration, are extremely difficult to perform accurately in the field and often require taking the unit to the shop. Even with frequent servicing, such units are often out of calibration and the resulting calibration errors lead to further reporting errors and increased service costs.
Another difficulty is created by the physical characteristics of pyroelectric crystals--namely that they produce an electrical potential only in response to changes in temperature. This characteristic requires that the infrared detector, that is, the pyroelectric cell, be subjected to changes in the amount of infrared striking it. In addition, the normal operating temperature of a railroad wheel bearing is determined relative to the ambient temperature. The requirement of measuring both the wheel bearing temperature and the ambient temperature provides a ready made opportunity to expose the pyroelectric cell to the required changes in the infrared heat signal. Difficulties arise, however, in choosing a suitable infrared source to determine the ambient temperature.
Some pyroelectric hot box detectors in the prior art approach this problem by merely leaving the detector turned on whenever a train is passing and aiming the lens so that it receives infrared from passing bearings, and from the undercarriage of the railroad cars. This passive-read system assumes that the temperature reading developed from looking at the undercarriage is the ambient temperature, and compares this to the temperature of the bearing. This solution works well if the undercarriage is actually at ambient temperature, but if, for example, the undercarriage is on fire (which not infrequently occurs from faulty brakes), such a detector will see the heat from the fire as the ambient temperature and will be unable to detect any problem with a bearing, or even to detect the fire itself. Less dramatically, the sensor may measure the heat from a spurious source, such as brakes, and, unable to distinguish between hot brakes and hot bearings, issue a hot box warning. Then the crew must stop the train, and walk the train searching for a non-existent over-heated bearing.
Another problem for passive-read systems is presented by the increased used of railroad spine cars, which are a skeleton steel-rail flatbed with trucks attached. Spine cars are used to haul semi-trailers piggyback. When a passive-read hot box detector looks at the undercarriage of spine cars, it is likely to take a "sky shot," and read only infrared from the distant sky as ambient. A sky shot temperature reading is usually about 20 degrees C. to 30 degrees C. less than actual ambient temperature. Naturally, this leads to many false warnings, since a bearing at normal operating temperature would show up as 40 degrees C. to 50 degrees C. hotter than the ambient temperature. Again, the crew must stop the train and walk the train searching for a non-existent hot box.
One prior art approach to overcoming this difficulty is to include a shutter that covers the lens at all times except when the apparatus expects to see a wheel bearing. This practice screens out all spurious infrared from overheated brakes and the like, and takes for its ambient temperature reading the temperature of the shutter blade inside the detector housing. The detector, however, warms up and cools down more slowly than the true ambient temperature, especially during periods of rapid ambient temperature changes. These changes predictably occur around sunrise and sunset, and unpredictably occur during weather changes and in magnitudes that depend on the season and the weather. The temperature inside the detector housing tends to lag the actual ambient temperature by about two hours. This temperature lag can cause the measured difference between the correct ambient temperature and the journal bearing temperature to be wrong by as much as 10 degrees C. In addition, sun loading can heat the detector unit to a temperature that is considerably hotter than the ambient temperature. These differences between internal detector temperature and the actual ambient temperature can obviously lead to erroneous comparisons between ambient temperature and bearing temperature, creating both false negatives and false positives.
In addition, the prior art shutter detection scheme requires synchronization between the opening and closing of the shutter and the passing of the bearings, i.e. the shutter must be open when a wheel is being scanned, and closed when no wheel is being scanned. This necessitates rapidly starting and stopping the shutter. The shutter is operated by an electric solenoid. The ancillary devices required to synchronize the movement of the shutter with the passing train wheels are complex and expensive. Repeatedly energizing the shutter solenoid wears out the solenoid quickly, and the jolt caused by stopping the shutter sometimes creates spurious signals from the pyroelectric cell due to its piezoelectric characteristics. Accordingly, although use of a synchronized shutter to screen unwanted infrared from the pyroelectric cell avoids the temperature sensing problems of the passive-read system, it leads to complex problems of its own.
Furthermore, prior art hot box and hot wheel detectors transmit an analog output signal. Analog signals are naturally more prone to degradation, distortion, and attenuation than digital signals, and typically can carry far less information. Increasingly, remote signalling devices and other ancillary equipment accept digital signals, which not only may convey more information, but do so more accurately than analog signals.
Therefore, a need exists for hot box and hot wheel detectors that are less expensive to manufacture, maintain, and operate; that are more reliable; that reduce or eliminate false negative warnings and false positive warnings, both of which are inordinately expensive; that produce consistent operating results over time by eliminating the effect of pyroelectric cell drift; that can generate either a digital or analog output signal, allowing the user railroad to use analog ancillary devices for their full useful life if desired, and then conveniently change to more modern digital ancillary devices; and that reliably measure ambient temperature notwithstanding spurious undercarriage or detector housing temperatures.